Cooperative binding systems are being studied taking into account site or subunit interactions, ligand interactions, aggregation and redistribution in proteins, and model systems. Methods are being developed to evaluate reasonable values for the parameters describing these systems. Changes in calcium binding to fibrinogen upon clotting by thrombin were investigated and correlated with the release of fibrinopeptides and the polymerization steps. Amino acid sequences of proteins are analyzed primarily with the Monte Carlo techniques to evaluate the uniqueness and similarity of these sequences. The property of uniqueness (the occurrence of a small peptide at a frequency considerably less than that expected) has been quantified, and speculations on this quantity and the immune response are under continued investigation.